A cold field emission electron gun or a thermal field emission electron gun typically comprises a tip having a radius of about 1 μm or less and an electrode with an aperture having a diameter of about 300-700 μM. If a high enough bias is applied between the tip and the electrode, then a sufficiently high electric field is generated at the tip to extract electrons from it to produce an electron beam.
The tip and the aperture need to be carefully laterally aligned and vertically spaced apart. If they are not properly aligned, the resulting electron beam can suffer from aberration and be misaligned with the optical axis. If they are not properly spaced, then extraction will not occur at workable voltages.
The tip is provided in a unit called an emitter which can be obtained commercially or made in-house. Most commercially available emitters, such as those from FEI Company (TM), USA and Denka (TM), Japan, have a cylindrical housing and the tip protrudes through an aperture located in the center of an end face of the housing.
The electron gun is constructed from an emitter and an electrode. The emitter and the electrode can be aligned before they are assembled into an electron beam system or aligned in-situ in an assembled electron beam system.
Where the emitter and the electrode are aligned before they are assembled into an electron beam system, the emitter is mounted on a moveable stage over the electrode. A skilled technician aligns the emitter with the aperture with the aid of a microscope. Once the technician is satisfied that the tip of the emitter and the aperture are aligned, he or she secures the emitter and the electrode in a support. The stage can then be removed.
Where the emitter and the electrode are aligned in-situ in the electron beam system, the emitter is mounted on a moveable stage. The emitter and aperture are aligned by moving the emitter and monitoring the beam.
Both types of gun take an appreciable amount of time and skill to align. Furthermore, where the emitter and the electrode are aligned in-situ, the movable stage forms part of the gun system, thus increasing its complexity.
The present invention seeks to provide a charged particle gun.